This project has as its main objective the study of a factor derived from murine T cells that assists other T cells to become cytotoxic. This humoral substance, termed killer assisting factor (KAF), is found in supernatants of mixed lymphocyte reactions (MLR) that are allogeneic at the major histocompatibility complex, allogeneic at the M1s locus and isogeneic. We have devised a new simplified method by which thymic cells in conjunction with exogenously produced KAF will stimulate the generation of killer T cells for cell-mediated lympholysis (CML). KAF was found to act upon T cells during the phase when pre-killer T cells become sensitized to alloantigens coded for by the MHC (H-2K and H-2D). We have found that the alloantigen and KAF must be experienced by the pre-killer cells early in the sensitization period. In addition, there is an apparent order in which the two signals may be received by the pre-killer cells, alloantigen first, followed by KAF. In this research project, we will delineate: (a) the requirements for generating KAF, (b) the characteristics of its cellular source, (c) its biological, (d) its biochemical properties, and (e) its mechanism of action.